Dithering systems have been used in a number of technologies in the past. The objective of a dithering system is to change a characteristic of a particular signal in a periodic (or random) fashion in order to provide a useful output. As is described in further detail, the dithering system of the invention may be used to change the relative location of an optical signal.
The present invention may be used with various types of displays and systems. Exemplary displays are a CRT (sometimes referred to herein as cathode ray tube) display, a liquid crystal display (sometimes referred to herein as "LCD"), especially those which modulate light transmitted therethrough, reflective liquid crystal displays, light emitting displays, such as electroluminescent displays, plasma displays and so on.
Conventional optical displays typically display graphic visual information, such as computer generated graphics, and pictures generated from video signals, such as from a VCR, from a broadcast television signal, etc.; the pictures may be static or still or they may be moving pictures, as in a movie or in a cartoon, for example. Conventional displays also may present visual information of the alphanumeric type, such as numbers, letters, words, and/or other symbols (whether in the English language or in another language). Visual information viewed by a person (or by a machine or detector) usually is in the form of visible light. Such visible light is referred to as a light signal or an optical signal. The term optical signal with which the invention may be used includes visible light, infrared light, and ultraviolet light, the latter two sometimes being referred to as electromagnetic radiation rather than light. The optical signal may be in the form of a single light ray, a light beam made up of a plurality of light rays, a light signal such as a logic one or a logic zero signal used in an optical computer, for example, or the above-mentioned alphanumeric or graphics type display. Thus, as the invention is described herein, it is useful with optical signals of various types used for various purposes. Therefore, in the present invention reference to optical signal, light ray, light beam, light signal, visual information, etc., may be used generally equivalently and interchangeably.
In an exemplary liquid crystal display sometimes referred to as an image source, there usually are a plurality of picture elements, sometimes referred to as pixels or pels, and these pixels can be selectively operated to produce a visual output in the form of a picture, alphanumeric information, etc. Various techniques are used to provide signals to the pixels. One technique is to use a common electrode on one plate of a liquid crystal cell which forms the display and an active matrix electrode array, such as that formed by thin film transistors (TFT), on the other plate of the liquid crystal cell. Various techniques are used to provide electrical signals to the TFT array to cause a particular type of optical output from respective pixels. Another technique to provide signals to the pixels is to use two arrays of crossed electrodes on respective substrates of an LCD; by applying or not applying a voltage or electric field between a pair of crossed electrodes, a particular optical output can be obtained.
One factor in determining resolution of a liquid crystal display is the number of pixels per unit area of the liquid crystal display. For example, Sony Corporation recently announced a 1.35 inch diagonal high resolution liquid crystal display which has 513,000 pixels arranged in 480 rows of 1,068 pixels per row.
Another factor affecting resolution is the space between adjacent pixels sometimes referred to "as optical dead space". Such space ordinarily is not useful to produce an optical signal output. The space usually is provided to afford a separation between the adjacent pixels to avoid electrical communication between them. The space also is provided to accommodate circuitry, leads, and various electrical components, such as capacitors, resistors, and even transistors or parts of transistors. The proportion of optical dead space to useful space of pixels that can produce optical output tends to increase as the physical size of the image source is decreased, for the space required to convey electrical signals, for example, may remain approximately constant although the actual size of the useful space of the pixels to produce optical output can be reduced because of anticipated image magnification. However, upon magnification of the image produced by such a miniature image source both the optical dead space and the useful optical space of the pixels are magnified. Thus, resolution tends to be decreased, especially upon such magnification.
The picture elements (pixels or pels) may be discrete pixels, blocks or areas where an optical signal can be developed by emission, reflection, transmission, etc. such as the numerous pixels in the miniature image source of Sony Corporation mentioned above. The optical signal referred to may mean that light is "on" or provided as an output from the device, or that the pixel has its other condition of not producing or providing a light output, e.g., "off"; and the optical signal also may be various brightnesses of light or shades of gray. Alternatively, the optical output or optical signal produced by a pixel may be a color or light of a particular color.
The pixels may be a plurality of blocks or dots arranged in a number of lines or may be a number of blocks or dots randomly located or grouped in a pattern on the display or image source (source of the optical signal). The pixels may be a number of lines or locations along the raster lines that are scanned in a CRT type device or the pixels may be one or a group of phosphor dots or the like at particular locations, such as along a line in a CRT or other device. The optical signal produced by one or more pixels may be the delivery of light from that pixel or the non-delivery of light from that pixel, or various brightnesses or shades of gray. To obtain operation of a pixel, for example, the pixel may be energized or not. In some devices energizing the pixel may cause the pixel to provide a light output, and in other devices the non-energizing of the pixel may cause the providing of a light output; and the other energized condition may cause the opposite light output condition. It also is possible that the nature of the light output may be dependent on the degree of energization of a pixel, such as by providing the pixel with a relatively low voltage or relatively high voltage to obtain respective optical output signals (on and off or off and on, respectively).
For example, in a conventional twisted nematic liquid crystal display device, polarized light is received by a liquid crystal cell, and depending on whether the liquid crystal cell receives or does not receive a satisfactory voltage input, the plane of polarization of the light output by the liquid crystal cell will or will not be rotated; and depending on that rotation (or not) and the relative alignment of an output analyzer, light will be transmitted or not. In an optical phase retardation device that has variable birefringence, such as those disclosed in U.S. Pat. Nos. 4,385,806, 4,540,243, and Re. 32,521 (sometimes referred to as surface mode liquid crystal cells), depending on the optical phase retardation provided by the liquid crystal cell, plane polarized light may be rotated, and the optical output can be determined as a function of the direction of the plane of polarization. In a CRT light emission or not and brightness may be determined by electrons incident on a phosphor at a pixel. In electroluminescent displays and plasma displays light output may be determined by electrical input at respective areas on pixels.
The interlacing of raster lines or display lines is a known practice used in television and in other types of display systems. For example, in NTSC and PAL television type cathode ray tube (CRT) displays it is known that two interlaced fields of horizontal lines are used to provide an entire image frame. First one raster or set of lines is scanned to cause one subframe (sometimes referred to as field) to be displayed; and then a second raster or set of lines is scanned to cause a second subframe (field) to be displayed. The electrical signals used to scan one line in one subframe and the electrical signals used to scan the relatively adjacent line of the subsequent subframe may be different, and, therefore, the optical outputs of those lines may be different. The two raster subframes are presented sufficiently fast that the eye of an observer usually cannot distinguish between the respective images of the two successive subframes but rather integrates the two subframes to see a composite image (sometimes referred to as a frame or picture). The two subframes are created sequentially by "writing" the image to respective pixels formed by phosphors to which an electron beam may be directed in response to electrical signals which control the electron beam in on-off and/or intensity manner. After the electron beam has reached the end of its scanning to create one subframe, e.g., the last pixel or phosphor dot area of that field, there is a period of time while the electron beam is moved or directed to the first pixel of the next subframe. During that period of time a blanking pulse is provided to prevent electrons from being directed to phosphors or pixels causing undesired light emission. Sometimes various circuits of a television or CRT display are synchronized to the operative timing of the television, CRT, etc. by synchronization with such blanking pulses.
The density of pixels, e.g., number of pixels per unit area, in a CRT display usually is, in a sense, an analog function depending on characteristics of the electron beam, drive and control circuitry for the beam, phosphor dot layout, shadow mask(s), etc., as is known. Usually a CRT is driven using the interlaced lines forming the subframes mentioned above. In an LCD, though, there is a fixed number of pixels per line or row; and data, e.g., whether a given pixel in a row is to transmit light or to block light transmission, usually is written to the pixels a row at a time. The data is written to one row, then to the next, and so on, and them usually is no interlacing of rows or of subframes as there is in CRT driving techniques.
In some LCD's the two subframes mentioned above usually are effectively averaged together, when driven by a CRT type of interlaced signal, since there usually is no physical interlacing of LCD pixels to form respective subframes as there are respective scan lines of phosphor dots, for example, in a CRT. Rather, the electrical signals for driving adjacent scan lines of different respective interlaced subframes of a CRT display, both usually are delivered to only a single row of pixels in an LCD. Each pixel responds to the electrical signal applied thereto to transmit or to block light, for example. Those two sets of electrical signals are applied to the row of pixels at different times. Therefore, at one time a given row of LCD pixels may present as an optical output optical information from one subframe and at a later time present optical information from the other subframe.
Since the optical information presented in one subframe is expected to be displaced in space from the optical information presented in the other subframe to obtain the interlacing pattern of a CRT display, careful examination of the optical output from the above-mentioned LCD will show an amount of "jittering" of the image. This jittering is caused by the pixels of one row periodically being changed so the optical output thereof sequentially displays the result of energization by signals representing one scan line of information from one subframe and then energization by electrical signals representing the adjacent scan line of information from the next subframe.
This jittering can degrade the displayed image and can make viewing uncomfortable. Also, the problems, such as viewing discomfort and/or image degrading, caused by jittering tend to increase as the image is enlarged or magnified, e.g., when the image is created by a relatively miniature image source, such as the SONY display mentioned above, and is magnified for direct viewing or for projection by a projector.
One technique for reducing the jittering is to use relatively slow liquid crystal display devices. Therefore, the liquid crystal display element or pixel tends to average the electrical signals applied thereto. A disadvantage to this technique, though, is that image resolution is reduced because the information representing two scan lines is combined into a single line. Also, a slow acting liquid crystal display element tends to have undesirable hysteresis that slows motion being shown by the display.
In a color display, such as a LCD (liquid crystal display), there usually are red, green and blue pixels which form a color triad (hereinafter referred to as triad). Sometimes the term of art "sub-pixels" is used as a label or nomenclature to refer to differently colored portions of a triad or of another group of pixels or portions of pixels. By operating the LCD in such a way that one or more of the pixels forming a triad provides (or produces) the respective color light of that pixel, different respective colors and white can be produced as output light. For example, if the red pixel of a triad were providing red output light; and the green and blue pixels were not providing output light, the light output from that triad would be red. Further, when two or more pixels of a triad are providing light output, a combination of those colors is seen by a person viewing (sometimes referred to as the viewer) the light output or image. The viewer usually visually superimposes the output light from the pixels of the triad; and the combined or superimposed lights therefrom provide the net effect or integrated light output of the triad. As an example, to produce a white light output from a triad, the red, green and blue pixels of that triad would provide, respectively, red, green and blue light; and those lights would be, in effect, superimposed by the viewer and seen as white light.
There is a continuing need and/or desire to improve resolution of displays. It also would be desirable to facilitate the placing of circuitry in a display while minimizing the optical dead space caused by the circuitry. There also is a need to reduce jitter.
In the above-mentioned patent applications are disclosed techniques for actively dithering, moving an optical signal, changing the location or optical path of an optical signal, etc. for several purposes, such as to increase resolution, to reduce jitter, and so on. There also are disclosed techniques for passive dithering, moving of optical signals, etc., for example to increase tie fill factor of an image provided by a display by expanding the image or pixels forming the image.
An LCD using the twisted nematic effect usually cannot switch between transmission states as rapidly as changes occur in the applied electrical signal which operates the LCD. For example, the electrical input to a twisted nematic LCD can change nearly instantly, but it takes a number of milliseconds for the LCD to respond dynamically to the change in electrical input to change the optical response of the LCD. When an LCD is used in a display system that employs dithering to double, quadruple or otherwise to change the effective number of pixels, for convenience hereinafter, sometimes referred to as optical line doubling (or OLD), the relatively slow response of the twisted nematic LCD compared to the faster operation of the dithering optics can result in an optical output that does not achieve the desired improvement in resolution or other optical effect.
The displaying of a dark scene using a display device (sometimes referred to as a passive display), which modulates light received from a separate light source, encounters a disadvantage which ordinarily is not present for displays which produce their own light, such as a cathode ray tube (CRT). The problem has to do with reduced resolution and/or contrast of the displayed image.
In a CRT, for example, when it is desired to display a dark scene, the intensity of the output light can be reduced. The different parts of the dark scene, then, all may be output at the reduced brightness or illuminance level. All pixels (e.g., picture elements, phosphor dots in a monochrome display or group of three red, green and blue phosphor dots for a multicolor display, etc.) of the CRT can be active so that resolution is maintained even though intensity of the light produced by the phosphors is reduced.
However, in a passive display device, such as a liquid crystal display, an electrochromic display, etc., whether of the light transmitting type or of the light reflecting type, the usual practice to reduce brightness of a displayed image or scene is to reduce the number of pixels which are transmitting light at a particular moment. Such a reduction reduces the resolution of the display. Also, such a reduction can reduce the contrast of the display.
The human eye has difficulty distinguishing between seeing or recognizing the difference between low and high brightness and contrast ranges. This difficulty is increased when the number of pixels is decreased and resolution is degraded.
It would be desirable to improve the contrast and resolution of passive displays.
In U.S. patent application Ser. No. 08/187,163 is disclosed a passive apparatus, such as an LCD, and method for displaying images with high contrast by controlling the light input to the display to control brightness of the output while operating respective pixels of the display to obtain good contrast substantially without regard to the output brightness. Different color effects also are disclosed using, for example, field sequential switching of respective color light. However, this is another example of a passive optical device, in this case an LCD, in which field sequential switching could be improved if coordinated with the delays inherent in the dynamic optical response of a liquid crystal cell, for example, relative to the changes in operating signal, such as electric field, voltage, etc.
As is described in application Ser. No. 08/187,163, an image of a candlelit room would be dim. In the prior art devices a relatively small number of pixels would be used, then, to transmit light to create the image, whereas a relatively large number of pixels would be used to block light transmission to give the effect of the reduced intensity or dim room. In the invention of such application, though, the number of pixels used to create the image remains constant, and the contrast ratio between one portion and another portion of the image remain constant; only the intensity of the illuminating light changes thereby to diminish the brightness of the room. Therefore, with the invention image data is not lost regardless of the brightness of the image, whereas in the prior art image data is lost because the additional pixels are used to brighten or darken the brightness of the image.
The features of the invention as described in that patent application can be used in a frame sequential basis. The features of the invention can be used regardless of whether the display is operated in reflective mode or in transmissive mode. Also, the features of the invention can be used in a virtual reality type display in order to provide a very wide range of contrast and of image brightness characteristics. The picture information is used to derive the brightness of the display, not the surrounding ambient. Using the invention of that application, the amount of information that can be conveyed by the display is substantially increased over the prior art. For example, if there were a grey scale of 100 shades of grey and a display with 10 shades of grey, the intensity of the illuminating source can be changed at 10 different levels, for example, and there also can be 10 different shades of grey provided by the display itself. Therefore, this provides 100 shades of grey. This characteristic can be increased by another factor of 10 by going to r, g, b (red, green, blue) modulation on a field sequential basis, which allows the possibility of 10 to the 6th different illumination levels and colors. The foregoing is especially important in head mounted displays where immersion in the image is extremely important. Using features of such patent application, there can be high illumination of the scene, then, the grey scale contrast ratio of the real image can be adjusted. As a result, there is a high contrast image presented in a bright motif. Another example using such invention is the ability to display a sunrise scene in which the red image is enhanced and the blue and green are minimized.
The invention of that application, then, can separate the two functions of brightness and image. The image is a function of the operation of the liquid crystal modulator and the illumination brightness is the function of the light source intensity. The r, g, b colors can be changed to give a candlelight or moonlight effect with good resolution and color function, but the brightness of the scene is a function of the background. As a result, it is possible to photograph the scene in daylight to get good contrast; and then by reducing the display illumination it is possible to give the impression of a moonlit or candlelit environment.